1. Field of the Invention
The present invention relates to an optical communication system, particularly to an apparatus for monitoring an optical signal passing through the optical communication system.
2. Description of the Related Art
In an optical communication system with a wavelength division multiplexing (WDM) method, an optical signal passing through the optical fiber generally includes a plurality of channels with wavelengths different from each other. In order to monitor certain characteristics, such as the optical signal-to-noise ratio (OSNR), wavelength and power with respect to each channel, the optical communication system includes an optical performance monitor (OPM).
Although a conventional OPM can measure various characteristics of an inputted optical signal, it does not provide a means or method that is capable of measuring a polarization characteristic. When an optical signal has a bad polarization characteristic, a polarization mode dispersion (PMD) may develop thus becoming a big problem to light transmission. The polarization mode dispersion occurs due to the relation between a physical property of an optical fiber and a polarization state of an optical signal passing through the optical fiber. In the polarization mode dispersion, a birefringence that may occur due to the noncircular ratio of a core, an asymmetric index of refraction, and an asymmetric stress in an optical-fiber drawing process cause group velocities of polarization components passing along two polarization axes to have differences, which result in a differential group delay (DGD), thereby causing the pulse to be spread. Accordingly, a polarization characteristic plays a large role in determining the characteristics of an optical signal.